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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
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0 replies
jlacosta
Apr 2, 2025
0 replies
interesting integral
Martin.s   0
27 minutes ago
$$\int_0^\infty \frac{\sinh(t)}{t \cosh^3(t)} dt$$
0 replies
Martin.s
27 minutes ago
0 replies
Inequalities
sqing   4
N 2 hours ago by sqing
Let $x,y\ge 0$ such that $ 13(x^3+y^3) \leq 125(1+xy)$. Prove that
$$  k(x+y)-xy\leq  5(2k-5)$$Where $k\geq 5.6797. $
$$  6(x+y)-xy\leq 35$$
4 replies
sqing
Yesterday at 1:04 PM
sqing
2 hours ago
Pre- Calc
AimlessNonsense   1
N 4 hours ago by Mathzeus1024
A rock got stuck in the tread of my tire and when I was driving 45 miles per hour, the rock came loose
and hit the inside of the wheel well of the car. How fast, in miles per hour, was the rock traveling
when it came out of the tread? (The tire has a diameter of 26 inches.)

I have been trying to figure this out for about 3 hours now and I know I am making some small mistake, but cannot seem to figure out what it is.
1 reply
AimlessNonsense
Aug 30, 2015
Mathzeus1024
4 hours ago
How to solve this problem
xiangovo   1
N 5 hours ago by loup blanc
Source: website
How many nonzero points are there on x^3y + y^3z + z^3x = 0 over the finite field \mathbb{F}_{5^{18}} up to scaling?
1 reply
xiangovo
Mar 19, 2025
loup blanc
5 hours ago
Inscribed Semi-Circle!!!
ehz2701   2
N 5 hours ago by mathafou
A right triangle $ABC$ with legs $AB = a$ and $BC = b$ is drawn with a semicircle inscribed into the triangle. What is the smallest possible radius of the semi-circle and the largest possible radius?

2 replies
ehz2701
Sep 11, 2022
mathafou
5 hours ago
geometry
carvaan   1
N 5 hours ago by vanstraelen
OABC is a trapezium with OC // AB and ∠AOB = 37°. Furthermore, A, B, C all lie on the circumference of a circle centred at O. The perpendicular bisector of OC meets AC at D. If ∠ABD = x°, find last 2 digit of 100x.
1 reply
carvaan
Yesterday at 5:48 PM
vanstraelen
5 hours ago
Finite solution for x
Rohit-2006   1
N 5 hours ago by Filipjack
$P(t)$ be a non constant polynomial with real coefficients. Prove that the system of simultaneous equations —
$$\int_{0}^{x} P(t)sin t dt =0$$$$\int_{0}^{x}P(t) cos t dt=0$$has finitely many solutions $x$.
1 reply
Rohit-2006
Today at 4:19 AM
Filipjack
5 hours ago
Inequalities
nhathhuyyp5c   1
N Today at 9:09 AM by Mathzeus1024
Let $a, b, c$ be non-negative real numbers such that $a^2 + b^2 + c^2 = 3$. Find the maximum and minimum values of the expression
\[
P = \frac{a}{a^2 + 2} + \frac{b}{b^2 + 2} + \frac{c}{c^2 + 2}.
\]
1 reply
nhathhuyyp5c
Yesterday at 6:35 AM
Mathzeus1024
Today at 9:09 AM
weird permutation problem
Sedro   2
N Today at 8:56 AM by alexheinis
Let $\sigma$ be a permutation of $1,2,3,4,5,6,7$ such that there are exactly $7$ ordered pairs of integers $(a,b)$ satisfying $1\le a < b \le 7$ and $\sigma(a) < \sigma(b)$. How many possible $\sigma$ exist?
2 replies
Sedro
Yesterday at 2:09 AM
alexheinis
Today at 8:56 AM
In a school of $800$ students, $224$ students play cricket, $240$ students play
Vulch   2
N Today at 8:12 AM by MathBot101101
Hello everyone,
In a school of $800$ students, $224$ students play cricket, $240$ students play hockey and $336$ students play basketball. $64$ students play both basketball and hockey, $80$ students play both cricket and basketball, $40$ students play both cricket and hockey, and $24$ students play all three: basketball, hockey, and cricket. Find the number of students who do not play any game.

Edit:
In the above problem,I just want to know that why the number of students who don't play any game shouldn't be 0, because,if we add 224,240 and 336 it comes out to be 800.I have solution,but I just want to know how to explain it without theoretically.Thank you!
2 replies
Vulch
Yesterday at 11:41 PM
MathBot101101
Today at 8:12 AM
Combinatorial proof
MathBot101101   8
N Today at 5:55 AM by MathBot101101
Is there a way to prove
\frac{1}{(1+1)!}+\frac{2}{(2+1)!}+...+\frac{n}{(n+1)!}=1-\frac{1}{{n+1)!}
without induction and using only combinatorial arguments?

Induction proof wasn't quite as pleasing for me.
8 replies
MathBot101101
Yesterday at 7:37 AM
MathBot101101
Today at 5:55 AM
Inequalities
sqing   25
N Today at 3:58 AM by sqing
Let $   a,b    $ be reals such that $  a^2+ab+b^2 =3$ . Prove that
$$ \frac{4}{ 3}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{11}{4 }$$$$ \frac{13}{ 4}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{2}{3 }$$$$ \frac{3}{ 2}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq -\frac{17}{6 }$$$$ \frac{19}{ 6}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2}$$Let $   a,b    $ be reals such that $  a^2-ab+b^2 =1 $ . Prove that
$$ \frac{3}{ 2}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }+ab \geq \frac{4}{15 }$$$$ \frac{14}{ 15}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }-ab \geq -\frac{1}{2 }$$$$ \frac{3}{ 2}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq \frac{13}{42 }$$$$ \frac{41}{ 42}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2 }$$
25 replies
sqing
Apr 16, 2025
sqing
Today at 3:58 AM
Three variables inequality
Headhunter   4
N Today at 3:18 AM by lbh_qys
$\forall a\in R$ ,$~\forall b\in R$ ,$~\forall c \in R$
Prove that at least one of $(a-b)^{2}$, $(b-c)^{2}$, $(c-a)^{2}$ is not greater than $\frac{a^{2}+b^{2}+c^{2}}{2}$.

I assume that all are greater than it, but can't go more.
4 replies
Headhunter
Yesterday at 6:58 AM
lbh_qys
Today at 3:18 AM
Indonesia Regional MO 2019 Part A
parmenides51   23
N Today at 2:08 AM by chinawgp
Indonesia Regional MO
Year 2019 Part A

Time: 90 minutes Rules


p1. In the bag there are $7$ red balls and $8$ white balls. Audi took two balls at once from inside the bag. The chance of taking two balls of the same color is ...


p2. Given a regular hexagon with a side length of $1$ unit. The area of the hexagon is ...


p3. It is known that $r, s$ and $1$ are the roots of the cubic equation $x^3 - 2x + c = 0$. The value of $(r-s)^2$ is ...


p4. The number of pairs of natural numbers $(m, n)$ so that $GCD(n,m) = 2$ and $LCM(m,n) = 1000$ is ...


p5. A data with four real numbers $2n-4$, $2n-6$, $n^2-8$, $3n^2-6$ has an average of $0$ and a median of $9/2$. The largest number of such data is ...


p6. Suppose $a, b, c, d$ are integers greater than $2019$ which are four consecutive quarters of an arithmetic row with $a <b <c <d$. If $a$ and $d$ are squares of two consecutive natural numbers, then the smallest value of $c-b$ is ...


p7. Given a triangle $ABC$, with $AB = 6$, $AC = 8$ and $BC = 10$. The points $D$ and $E$ lies on the line segment $BC$. with $BD = 2$ and $CE = 4$. The measure of the angle $\angle DAE$ is ...


p8. Sequqnce of real numbers $a_1,a_2,a_3,...$ meet $\frac{na_1+(n-1)a_2+...+2a_{n-1}+a_n}{n^2}=1$ for each natural number $n$. The value of $a_1a_2a_3...a_{2019}$ is ....


p9. The number of ways to select four numbers from $\{1,2,3, ..., 15\}$ provided that the difference of any two numbers at least $3$ is ...


p10. Pairs of natural numbers $(m , n)$ which satisfies $$m^2n+mn^2 +m^2+2mn = 2018m + 2019n + 2019$$are as many as ...


p11. Given a triangle $ABC$ with $\angle ABC =135^o$ and $BC> AB$. Point $D$ lies on the side $BC$ so that $AB=CD$. Suppose $F$ is a point on the side extension $AB$ so that $DF$ is perpendicular to $AB$. The point $E$ lies on the ray $DF$ such that $DE> DF$ and $\angle ACE = 45^o$. The large angle $\angle AEC$ is ...


p12. The set of $S$ consists of $n$ integers with the following properties: For every three different members of $S$ there are two of them whose sum is a member of $S$. The largest value of $n$ is ....


p13. The minimum value of $\frac{a^2+2b^2+\sqrt2}{\sqrt{ab}}$ with $a, b$ positive reals is ....


p14. The polynomial P satisfies the equation $P (x^2) = x^{2019} (x+ 1) P (x)$ with $P (1/2)= -1$ is ....


p15. Look at a chessboard measuring $19 \times 19$ square units. Two plots are said to be neighbors if they both have one side in common. Initially, there are a total of $k$ coins on the chessboard where each coin is only loaded exactly on one square and each square can contain coins or blanks. At each turn. You must select exactly one plot that holds the minimum number of coins in the number of neighbors of the plot and then you must give exactly one coin to each neighbor of the selected plot. The game ends if you are no longer able to select squares with the intended conditions. The smallest number of $k$ so that the game never ends for any initial square selection is ....
23 replies
parmenides51
Nov 11, 2021
chinawgp
Today at 2:08 AM
RREF of some matrices
tommy2007   4
N Apr 6, 2025 by tommy2007
for $\forall n \in \mathbb{N},$
what is the maximum integer that appears in one of the Reduced Row Echelon Forms of $n \times n$ matrices which has only $-1$ and $1$ for their entries?
4 replies
tommy2007
Apr 2, 2025
tommy2007
Apr 6, 2025
RREF of some matrices
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tommy2007
266 posts
#1
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for $\forall n \in \mathbb{N},$
what is the maximum integer that appears in one of the Reduced Row Echelon Forms of $n \times n$ matrices which has only $-1$ and $1$ for their entries?
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tommy2007
266 posts
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I'm kinda sure that this problem won't have a general answer ( :( ), but I'll just leave this one here as an archive :)
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paxtonw
28 posts
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We start with an n by n matrix made of only -1 and 1. We do row operations to get it into reduced row echelon form (RREF). The biggest number that can show up is the nth Fibonacci number, because you can build a matrix where the numbers grow like the Fibonacci sequence by adding rows together. So the number Fn can appear. Also, you can’t get anything bigger. Row operations just add and scale rows, so the numbers can’t grow faster than Fibonacci numbers if you start with only -1 and 1.

So the answer is Fn, and nothing bigger is possible.

Please let me know if you require more explaination.
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tommy2007
266 posts
#4
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paxtonw wrote:
We start with an n by n matrix made of only -1 and 1. We do row operations to get it into reduced row echelon form (RREF). The biggest number that can show up is the nth Fibonacci number, because you can build a matrix where the numbers grow like the Fibonacci sequence by adding rows together. So the number Fn can appear. Also, you can’t get anything bigger. Row operations just add and scale rows, so the numbers can’t grow faster than Fibonacci numbers if you start with only -1 and 1.

So the answer is Fn, and nothing bigger is possible.

Please let me know if you require more explaination.

For an counterexample, the case $n=4$ has $57$ different RREFs (Check the uploaded file) and the maximum integer among them is $1$ :(
Any more ideas? :o
Attachments:
rref_matrices.tex (6kb)
This post has been edited 3 times. Last edited by tommy2007, Apr 2, 2025, 1:54 PM
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tommy2007
266 posts
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\bump :)
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